The present invention relates to improvements in apparatus primarily designed to measure the acoustic impedance or admittance of structures more particularly, it relates to apparatus designed to make such measurements for the human external auditory canal.
In prior art, the measurement of the acoustic impedance or the measurement of the reciprocal acoustic admittance has typically involved the measurement of such quantities at relatively low frequencies where each acoustic element was believed to be relatively small compared to the wavelength of sound and it could thus be treated as a lump constant, such as mass or stiffness. This is similar to describing electrical structures which are measured as having the value of capacitance or inductance rather than being parts of distributed circuits or transmission lines. In the present invention, the type of transducers typically employed in the prior art are electromagnetic transducers of the moving iron type because these are relatively efficient and can be produced with relatively small dimensions for the purpose. The major disadvantages of such devices has been their nonlinearity because they were often employed not only for the purpose of transmitting measuring signals the voltage of which was measured across the terminals of the transducer, but also simultaneously to produce additional acoustic signals in the ear to measure the reaction of the human muscular system and nervous sytem when exposed to acoustic stimuli.
This problem has caused severe measurement interaction because of the non-linearity of these transducers. A further problem has been that the characteristics were extremely variable from unit to unit so that non-linear compensating circuits could not be employed.
In the present invention, these disadvantages have been overcome by a rather simple circuit based on an electromechanical analysis of the transducer and probe circuits themselves.